A fuel cell system obtains electric energy by electrochemically reacting a combustion gas such as hydrogen with an oxygen-containing oxidation gas via an electrolyte. Since such a fuel cell system has a defined operating temperature that allows for electric power generation, it is sometimes arranged such that, if the temperature of the fuel cell has not reached the operating temperature, e.g. at the time of start-up, the fuel cell is heated up (warmed up) quickly to reach the temperature that allows for electric power generation. Furthermore, since generating efficiency of the fuel cell system is decreased when the operating temperature is too high, it is arranged such that, if the temperature of the fuel cell is too high, heat exchange means such as a radiator is used to exchange heat with outside air to cool down heat medium, and then the cooled heat medium is used to cool down the fuel cell.
One type of conventional fuel cell systems is a fuel cell system in which a fuel cell is warmed up by using heat medium that has been heated through a heater of fuel combustion-type and an exhaust gas obtained from such a heater (see Japanese Patent Application Laid-Open Publication No. 2001-155753, for example). With this type of fuel cell system, the fuel cell can be warmed up quickly by using the heated heat medium and the exhaust gas even at the time the combustion-type heater is just after start-up and the temperature of the heat medium is still low.
Another type of conventional fuel cell systems is a fuel cell system in which heat medium is heated by using heat of a hydrogen combustor for burning hydrogen exhausted from a fuel cell and the heated heat medium is used to warm up the fuel cell, where intermediate heat exchanging means is disposed downstream from a heat exchanger between the hydrogen combustor and the heat medium, in which the heat medium is caused to circulate, so that the heat of the hydrogen combustor is released via the intermediate heat exchanging means (see Japanese Patent Application Laid-Open Publication No. 2004-235075, for example). With this type of fuel cell system, the heat of the hydrogen combustor can be released via the intermediate heat exchanging means, and heating of the heat medium in thermal contact with the hydrogen combustor can be reduced. Therefore, it is possible to prevent excess temperature rise in the heat medium for cooling down the fuel cell.
In addition to the above-mentioned documents, Japanese Patent Application Laid-Open Publication No. 6-304087, Japanese Patent Application Laid-Open Publication No. 1-169269, and Japanese Patent Application Laid-Open Publication No. 2003-249251 also disclose techniques related to fuel cell systems.